A drag of a topic...

As I mentioned in previous posts, this year's TARC competition will probably see many teams struggling to loft their two eggs to altitude without busting the weight limit on the rocket. Most teams use the Tim Allen philosophy when it comes to increasing altitude - use a bigger motor (more power!). But there is another way, one that can result in a gain of tens of feet in the rocket peak altitude (maybe as much as a hundred). This approach, used by contest rocketeers all over the world, is largely ignored by TARC teams because it adds considerable work and time in building the rockets. Drag reduction can't be done in 30 minutes, but the math and years of experience shows that it can produce significant improvement if done properly.

Long before we had personal computers, a lot of smart folks were looking at drag and its effects on rocket flight. The math is fairly complicated - at a basic college calculus level - but even back in the 60's and 70's, advanced rocketeers were producing tables and formulae to estimate the amount of drag a model rocket would experience in flight. A oversimplification would characterize the drag by a single dimensionless number, Cd, called the drag coefficient. A flat surface has a drag coefficient well above 1, whereas a sphere has a Cd around 0.45. Most of my rockets seem to have drag coefficients around 0.75 - at least that's what I get when I try to match the simulations to the altitude profile given by the altimeters. The deal with drag is that you want to keep the airflow smooth around the rocket - any disruption, whether it is caused by rail buttons, base drag at the rear of the model, rough finish, or improper match of the nose cone to the body tube, increases the drag coefficient and reduces the altitude. TARC rockets need rail buttons or launch lugs (unless you want to spend the bucks to build a launch tower), so not much can be done there. However, every rocket can have a smooth finish - all that takes is additional work.

Rocksim screen capture showing Cd override panel (Click to enlarge).

To get an idea of how altitude depends on drag coefficient, I fired up my copy of the Rocksim software and loaded a past TARC design, the Over Easy. Rocksim has several advantages over Open Rocket in the technical area - you can design models with external pods and fins on fins, you can run simulations to figure out the optimum rocket weight, and you can put in your own values of the drag coefficient for the model (Open Rocket lets you do something similar by allowing you to choose the component finish - rough, unfinished, regular paint, smooth paint, and polished - but you just can't put in a number). I kept everything constant (weight, motor, etc)  except for the drag coefficient, which I varied from 0.55 to 1.05. The results are below - note that the altitudes range from 730 to over a thousand feet!

Effect of varying the Cd.

The catch is that a smooth finish requires paint, which adds weight to the rocket. So, as with anything in the real world, there is a tradeoff; you want as smooth a finish as possible without adding so much weight that you eliminate the altitude gained by reducing the drag. Experience has shown that a one or two primer coats, each followed by a vigorous sanding with increasingly fine grit sandpaper, and then a base coat of paint can yield a pretty smooth finish with only a modest weight increase.

Back in 1970, Estes produced Technical Report TR-11 "Aerodynamic Drag of Model Rockets". I very much recommend that all TARC teams and serious rocketeers read this, even though it is filled with math and stuff. At a very minimum, they should read pages 39-47, where there are some simple guidelines for drag reduction and flight test data for various configurations and finishes for an Estes Alpha. By airfoiling the fins, adding a boat tail, and putting a smooth finish on the model, they were able to improve the Alpha's measured altitudes from 319 to 446 feet, which is huge! This is a increase of about 30%, which is in line with the Rocksim simulations I performed for the Over Easy.

In case you can't locate a copy of TR-11 online (Google "Estes TR-11 drag"), here are the 5 rules for drag reduction:

• Rule 1: USE GOOD WORKMANSHIP

From the beginning of model construction, take time to sand all parts for a good fit: match the nose cone - body tube junction carefully, round the leading edge and sharpen the trailing edge of the fins. This initial work is a great step toward the reduction of the pressure drag of the rocket.

• Rule 2: ALIGN FINS AND LAUNCH LUGS PROPERLY

Correct fin alignment will keep the model from rolling during the ascent; this will eliminate unnecessary induced drag caused by the fins twisting through the air at angle of attack. Misalignment of the launch lug can cause flow separation on the body tube and excessive pressure drag.

• Rule 3: PUT A SMOOTH FINISH ON THE MODEL

Besides giving a high quality appearance to the model, the smooth finish delays transition of the flow to the high drag turbulent boundary layer condition. Even when the flow is turbulent, a slick surface will have less drag than a rough surface; so to reduce skin friction drag get the model rocket finish mirror-smooth.

• Rule 4: FILLET THE FINS

Reduce the interference drag by filling in the fin-body tube junction to guide the air smoothly past the fins.

• Rule 5: BOAT-TAIL WHENEVER POSSIBLE

Anytime the body tube diameter is greater than the engine diameter because of some special design feature, boat-tail the model. The base drag, which contributes an appreciable fraction of the total drag will be cut drastically by the boat-tail.

The moral of this post? A slick rocket is a good thing.

Rockets for the 4th...

For the past few years, I have tried to commemorate the 4th of July by building a rocket with a patriotic theme during the month. I usually start in late June with the intent of finishing by the 4th, but somehow things always come up - travel, weather, cosmic rays, whatever. This year it was business travel, and a lot of it; I have been away for nearly half the month, which doesn't leave much time for rocket building (or blog posts). However, I did finish the rocket earlier this week, keeping alive the tradition of getting the model built by the end of July. So I guess I will need to start a build in early June if I want to get it done by the 4th, or change my expectations to completing the rocket by July's end.

This year's choice was the Quest America, a simple four fin rocket that has been around for a few years (It was in the 2010 catalog). It was an easy build, going together quickly, and the two color paint job (blue fins, white body and nose cone) was not hard to do with Tamiya tape. One of my main gripes with Quest kits is that they feature "peel n cuss" stickers, which are a great source of annoyance - they look terrible and are difficult to handle. I much prefer water slide decals, so I replaced the white star fin stickers with Estes Comet decals that I ordered from Excelsior. I tried to replace the main body sticker with a waterslide decal I made on a color laser printer, but the decal was so long it kept tearing. This forced me to use the Quest sticker, which required some trimming (and cursing) to fit properly. I must confess the rocket doesn't look too bad; the white body tube seems to mitigate some of the shiny associated with peel n stick.

Here's the beauty shot:

Quest America (Click to enlarge).

And while we are on the subject of this year's patriotic model, here are a few from the past:

Estes Liberty (2011 - Click to enlarge).

Citation Patriot (2012 - Click to enlarge).

Estes Yankee (2013 - Click to enlarge).

Centuri Screaming Eagle (2014 - Click
 to enlarge).

2015 patriotic rocket done - back to other builds.

Thoughts on a beginner rocket...

I am often asked what the best rocket is for a beginner, or other variants of this like:

• What is a good rocket for me and my kid to build?
• What rocket would you recommend for a school (cub scout, girl scout, 4H) group?
• I (and/or my kid) want to fly at your launch this weekend. What can I get?

The answer, of course, depends on what you wish to achieve. Do you

1. Want to learn/teach the basics of rocket building?
2. Want something for a one-time launch, and have plenty of other things to do?

If you fall into #2, the answer is easy. Pick up a RTF (Ready To Fly) rocket at Hobby Lobby, Hobby Town, Michaels, or a local hobby shop. Estes makes quite a few of these, ranging from standard 3 fins and a nose cone (3FNC) models to colorful crayon rockets - you can find a few listed here.

If you are of the category #1 persuasion, you have many, many options. If you are building the rocket solo, or with another family member, the important thing to consider is how much time you want to spend in the build. Messing with wood and paint is rewarding, but it does take time - not good if you want to fly tomorrow morning. Fortunately, there is a whole line of rockets produced by Estes and Quest that feature plastic parts that are easily assembled in 30 minutes or so. The Estes Alpha III is my favorite; their Generic kit is also nice. One of the more unusual kits is the Dragonite, which snaps together, requiring no glue - there were quite a few of these at a recent cub scout launch. Quest has their line of Quick Kits, with the Astra III being the most popular. Any one of these will have you ready to fly in under an hour, and you will still learn the basic parts of the rocket.

These rockets are also good for group builds and you can buy them in bulk. I get many of the Alpha III's from AC Supply, where you can get 12 for $72 ($6 per rocket). Quest also offers several bulk packs featuring the Astra III, Starhawk, and other fast builds.

Alpha III bulk pack (Click to enlarge).

Those of you wishing to take your time and enjoy the full experience of rocket building should choose a level 1 kit. There are TONS of these, the most venerable and well known being the Estes Alpha. Practically every rocket company out there - Estes, Quest, Semroc/eRockets, Fliskits, Aerospace Specialty Products, Custom, Sunward, Squirrel Works, Balsa Machining Service, Pratt Hobbies, etc., etc. - offers level 1 kits. Pick one; you can't go wrong. It'll take you an hour or so to build the rocket, and a few hours for the glue to dry. The painting and finishing is a matter of preference - many rockets are flown "nekkid"for the first time; some rocketeers feel this is a way for the model to earn its paint. I like to spend days getting a nice finish; most models fall in between "nekkid" and "catalog quality".

I have always been partial to the Estes Alpha, as I have built a zillion of these over the years. A new favorite is the Balsa Machining Service's School Rocket.  At $5.25 a model, it's cheap, has quality parts (laser cut balsa fins and slotted body tube, streamer, balsa nose cone), and goes together very nicely. Great for first timers, and also really good for groups who have an hour or more in their build session.

Estes Alpha (click to enlarge).

BMS School Rocket (Click to enlarge).

There are plenty of rockets out there - pick the one you like, and have fun!

A different type of ORC...

For several years, Essence's Model Rocket Reviews & Resources (EMRR) sponsored online rocket contests through their website. Some of these were virtual, using Rocksim or other design programs to create a rocket that would meet a specific set of virtual challenges; others involved actually building a rocket to match photos of SciFi spaceships or from a box of junk shipped to the contestants (one of my favorites - the Box O' Parts). Despite the excitement of TARC (and now Geezer TARC), I miss these competitions very much, and have been waiting patiently since the last one in 2010 for someone or some group to bring them back. I kinda hoped that the NAR (National Association of Rocketry) would do something similar when they revamped their website, but alas, it did not happen. A few days ago, I decided that I had had enough, and made up my mind that I, rocket nerd of a great many years, would bring them back on my own. Heck, I'm even going to put up the first set of prizes.

Which meant I had to do some thinking - on a weekend, no less. A name for the contest series was required, and, given my personality and those of some my online colleagues, I thought of Old Grouch Rocketeer Events, or O.G.R.E.s. This had a nice ring, but I already have enough of a rep, so I settled on something simpler - the Olde Rocketeer Contests, or O.R.C.s. This worked perfectly, as I planned to based the contests out of Ye Olde Rocket Forum (YORF), where the owner generously carved out a section for the competitions. With the series name and hosting location decided, I then had to conjure some contest ideas. Two came to mind fairly easily, and so I posted my first rocket contest.

I'm excited about the O.R.C.s, and want as many people to participate as possible. If you want to join the fun, head over to YORF, sign up if you are not a member, and mosey to the Olde Rocketeer Contests section. That is where you can get the official rules, the Q & A, and see the entries as they are posted. You can also send me ideas for future ORCS if you wish - I can use all the help I can get! And while you are there, you might check out the other parts of YORF; there's some good information and ideas in the threads, if you can get past the old guys talking about politics and poop.

So, in the spirit of the old EMRR, here is the first O.R.C.:

O.R.C. #1 - Delivering the mail

Remember the days when you gazed at that special rocket in the Estes or Centuri catalog day after day, carefully saving your allowance until you had enough money to buy it? Do you remember filling out the order form, giving the money to your parents to get a money order, and the excitement of mailing these things off, knowing that that rocket would be yours in a few weeks? Do you recall the rush you felt upon coming home from school and checking to see if that rocket had finally arrived? You knew it would, of course - the mail was always delivered, even if it was slow in those days. Today’s youngsters have it easy - priority and overnight mail with tracking numbers take away all the suspense, frustration, and elation upon delivery we felt back in rocketry’s first decade or two. We can’t return to the past, but we can recapture a bit of the excitement and suspense in this first ORC.

Can you deliver the mail?

The rules are pretty simple:

1. Design and build a stable rocket powered by one or more 18 mm motors. This rocket must include as part of its structure some object used in the shipping of letters or parcels by the Post Office - mailing tube, shipping box, envelope, etc
2. Document the build (and the flight) on a thread in this forum.
3. Take a “glamour shot” of the rocket so we can all admire your handiwork.
4. Make at least one flight of the model, marking this flight with pictures (minimum of one pad shot and one showing the rocket lying on the ground (or in a tree) at the end of the flight.
5. Only 1 entry per contestant, please. Contest deadline is 11:59 PM Central Daylight Time on August 12, 2015 (the 45th anniversary of the Postal Reorganization Act).

How to participate:

a) Send a PM on YORF to Vanel or an email to cookewj [at] comcast.net stating your intent to enter the contest.
b) Create a thread in the Contest Submission section containing the following items before the contest deadline:

• Open Rocket or Rocksim design file. Traditional Geezers may submit a PDF or scanned image of a paper plan showing all dimensions (including CG/CP locations) and a parts list.
• Post (hopefully multiple posts) documenting the build and flight.
• The “glamour shot."
• The two flight images described above (more if you wish, but please, no more than 5).

Scoring:

Design (25 points maximum):

• 15 points for Open Rocket file, Rocksim file, or paper plan/parts list
• 5 points for use of mailing materials in every major component (body tube, fins, nose cone)
• 5 points for clustered motors or multiple stages

Documentation (25 points maximum):

• 10 points for build and flight documentation thread
• 5 points for “glamour shot"
• 5 points for pics of rocket on the pad and just after landing
• 5 points for image of rocket in powered flight and/or descending via recovery device

Flight (25 points maximum)

• 25 points for stable flight

Best looking (25 points maximum - determined by vote of the YORF membership):

• Number of points = [Sum of (# entries + 1 - voter rank)] X 25 / (# of voters x # entries), where voter rank is 1 for first place, 2 for 2nd, and so forth. The sum is over the # of voters.

Winner is the one whose entry has the most points.

Prize table:

• $60 eRockets gift certificate 
• Estes Ventris 
• Estes Altimeter
• Semroc Centaur
• Squirrel Works Red Baron

Cub scout launch...

Scouts prep their rockets for launch (Click to enlarge).

Every year, a former president of my rocket club, Chuck Pierce, hosts a launch for some local cub scouts. I love helping him with these events - the scouts are a fine organization, and deserve our support whenever possible. The launch was held today, and unlike other activities this year, I was actually in town to assist - even if it did mean leaving the apartment at an unnatural 7:15 AM on a Saturday morning. Two packs of cubs - Packs 351 and 83 - were expected to be in attendance, so I anticipated a fair number of rockets taking to the sky.

Chuck sends an Estes Crossbow on its way (Click to enlarge).

Chuck and I arrived at the Space Camp launch field around 7:25, and were almost immediately joined by Woody Bevill, who showed up all cool like on his big motorcycle. The three of us had the range set up in just under thirty minutes - 4 racks, with 3 pads per rack, all tied into the trusty HARA launch controller. The Space Camp field is way too small for rocket flying, as evidenced by the bazillion rockets decorating the surrounding trees (Woody said it looked liked the they were stricken with a plague); however, the wind was light and I figured we wouldn't loose too many, as long as the kids stuck with A and B impulse motors. It turns out that we did have to tilt the pads a bit more to the north after loosing a couple in the first set of launches. This helped significantly, though there was one RV in the camper park south of the field that seemed to be ground zero for the day - at least two rockets landed on its roof.

Woody launches a "white and green rocket" - actually an Estes L.G.M.
(Click to enlarge).

The cubs were ready to fly around 8:15 AM, and we put up rack after rack of rockets; I estimated 7 sets of 12, or around 80ish. This was done in just over 90 minutes, which means rockets took to the air at the impressive average rate of one per minute - Chuck and Woody are no slouches when it comes to handling the LCO (Launch Control Officer) duty. We all helped the scouts get their rockets ready on the pads, and I hung back and took pictures while Chuck and Woody pressed the Fire button. As far as the rocket flying, it was pretty much as expected - lots of good flights, a few (3?) in the trees, one coming in ballistic for a core sample, and the occasional rocket malfunction (fin coming loose, parachute tangling or stuck in body tube). A goodly amount of fun was had by all, which is the most important part of rocketry. However, I do think my colleagues need a lesson in model rocket kit identification - "little red rocket", while descriptive, is not very distinctive. To be fair, this talent is not normally needed, as the LCOs at our club launches have a flight card for each model on the pad, which has the rocket name, flyer, motor, etc. At last year's scout launch, there were a lot of Alpha III's present; there were also some this year, though the Estes snap together Dragonite was equally popular.

Activity around the pads (Click to enlarge).

We broke down and stowed the equipment in short order. Chuck and I were eating a quick late breakfast at Shaggy's by 10:15, and I was back at the apartment 30 minutes after that. Rockets and food make for a very good start to a Saturday.

An Estes Crossfire leaves the pad on a B motor - note the
3 Dragonites on the rack to the right (Click to enlarge).

An Alpha heads into the blue (Click to enlarge).

Two rockets under parachute (Click to enlarge).

An Estes Red Rider misses the trees (Click to enlarge).

Estes Power Patrol descends to Earth (Click to enlarge).

A potentially dangerous boo-boo

One of the great things about social media is that we can post pictures of our rocketry-related activities for the enlightenment and approbation of all (yes, I can use big words when I want). I was cruising Facebook recently, and noticed a pic someone had posted of a 3 motor cluster he was building from parts. Look at the image below - notice what's wrong?

If you realized the grain in the balsa fins is running the wrong direction, you would be right. One of the cardinal rules of rocketry is that you never, never, never, never lay out and cut wood fins so that the grain runs parallel to the body tube.  In this orientation, the fins are very weak, and easily snap at the slightest force (think breaking off a piece of a Kit-Kat candy bar). This was very quickly pointed out to the builder, and he sought to rectify his error by laminating the fins with paper, While this will add some strength, it will not be enough; after all, this is a cluster of 24 mm motors, and 3 E12's in this puppy will be the equivalent of a F36. Can you say that this rocket will leave the pad like a bat out of Hades? At high speed, even papered balsa fins will "flutter". For those of you who have never seen fin flutter, here is a YouTube vid of a high power fiberglass rocket going supersonic; the flutter occurs at max speed, about 10-11 seconds into the video:

So guess what is going to happen when that 3 engine cluster gets some speed? Papered or no, the fins will flutter and snap along the grain - large amounts of fin area will fall away, and the rocket will go unstable. NOT the thing you want to happen at a launch, especially if there are lots of bystanders.

The proper way to make/attach fins is with the grain parallel to the leading edge. In this orientation, the fins will flex, but the grain is (almost) perpendicular to the force. Of course, your rocket could be going so fast that the fins shred no matter how the grain is oriented (a common mishap with supersonic attempts), but it will take much more force to break them. So, when you build a rocket, orient the grain of wooden fins parallel to the leading edge, as depicted in the below image of the Balsa Machining School Rocket (currently under construction).

This is one of the things I check when I am acting as a RSO (Range Safety Officer) at a launch. I have seen too many instances of fins shredding due to wrong grain orientation; in one case, a guy had to dodge a rocket after two of its fins broke in pieces right after leaving the pad. Unfortunately, some RSO's don't pay much attention to the fin grain; even painted, one can wriggle the fins to check the strength, so it is not hard to do.

Parallel grain is just not poor rocket construction - it's unsafe.

Weight is everything...

As I begin construction of my first Geezer TARC rocket, I find myself confronted by a sobering aspect of this year's competition... Weight.

The TARC rules specify that the rocket may weigh no more than 650 grams fully loaded on the pad. This means that you have to pack 2 eggs, the protection for those eggs, altimeter, parachute, wadding (or dog barf or baffle), shock cord, and the motor(s) into a rocket that will loft those eggs to 850 feet and bring them safely to the ground at a descent rate of around 21 feet per second. At first, I thought "no problemo"; after all, I build light, and I am usually well below - sometimes by 50% - the weight limit. But as the designs, both simple single motor and complex cluster, developed, I found that weights under 600 grams were difficult to achieve - and that was with little egg protection. Why is this?

1) The rocket has to withstand hitting the ground at 21-22 feet per second. Doesn't sound terribly fast, but this is a 15 mile per hour impact speed. You are going to have to build robustly if you want the model to last more than one flight, especially if the ground is not nice soft grass. This means basswood/plywood fins and thick body tubes - all of which are heavier than standard body tubes and balsa fins.

2) The eggs have to survive the impact with nary a crack. They are going to have to be protected well, which means a decent amount of weight devoted to padding. This is the big unknown in my designs - I do not yet have a feel for how much this will weigh. I do know that it cannot be much more than 50 grams, or my rockets will bust the weight cap. Time for some research and experimentation. But this brings up a good point - TARC teams and mentors are going to have to pay careful attention to weight in this year's competition, as it will be very easy to exceed the 650 gram limit.

As I ponder egg protection, I have made good progress on my first entry - a 4 engine cluster I call "Nemesis", after the Greek goddess of revenge. Hopefully, she will not end up too fat to fly.

Open Rocket simulation of Nemesis under power.